An aluminum alloy extruded tube or a tube obtained by bending an aluminum alloy sheet material has been used as a refrigerant tube (refrigerant passage) for an aluminum heat exchanger that is joined by brazing. Such a refrigerant tube is designed so that a sacrificial anode effect due to a Zn diffusion layer is achieved by thermally spraying Zn onto the side of the extruded tube that forms the outer side of the refrigerant tube, or cladding the side of the tube (obtained by bending an aluminum alloy sheet material) that forms the outer side of the refrigerant tube with an Al—Zn-based alloy (sacrificial anode material) in order to improve the corrosion resistance of the outer side (that comes in contact with the atmosphere) of the refrigerant tube.
In recent years, a reduction in thickness and an improvement in corrosion resistance have been required for materials used to produce an automotive heat exchanger, and a decrease in corrosion rate of the sacrificial anode layer through a reduction in Zn content in the sacrificial anode material, and an increase in thickness of the sacrificial anode layer have been desired. However, it is difficult to reduce the amount of Zn thermally sprayed to the extruded tube from the viewpoint of thermal spraying efficiency. When using the tube obtained by bending an aluminum alloy sheet material, since the potential of the sacrificial anode material increases due to the effects of the diffusion of Cu included in the core material (Al—Mn—Cu-based alloy), and it is difficult to achieve a potential difference sufficient to obtain the sacrificial anode effect when the amount of Zn is reduced, it is difficult to reduce the Zn content in the sacrificial anode material (see FIG. 3). It is difficult to increase the thickness of the sacrificial anode layer by increasing the cladding ratio from the viewpoint of production cost.
A brazing sheet that is provided with a potential gradient so that the potential increases from the outer side toward the inner side after brazing by increasing the Cu content in the filler metal situated on the inner side as compared with the core material, a brazing sheet that is designed so that the potential increases from outer side toward the inner side through a concentration gradient of Zn and Cu that is formed by adding Zn to the filler metal situated on the outer side, adding Cu to the filler metal situated on the inner side, and adjusting the ratio of Zn to Cu within a specific range, have been proposed. However, since a layer that is formed by Cu diffused from the filler metal and has a higher potential is thin, and the potential difference between the layer having a higher potential and the core material is small, most of the core material is consumed due to corrosion, and the effect of suppressing the occurrence of a through-hole is insufficient immediately before a through-hole occurs.
An aluminum alloy clad material that is designed so that the inner layer (that is situated on the inner side of a heat exchanger and comes in contact with a refrigerant) is prevented from melting during brazing by adjusting the Si content in the inner layer to 1.5% or less, has also been proposed. However, since the Cu content in the core material is high, Cu diffuses into the outer layer during brazing, and causes the sacrificial anode effect of the outer layer to deteriorate. Moreover, since the potential of the core material is significantly higher than that of the outer layer, the outer layer is consumed at an early stage.